Thyroxine binding globulin (TBG) is the principal serum carrier protein for thyroxine (T.sub.4) and triiodothyronine (T.sub.3). The physiological role of TBG is best understood in the context of T.sub.4 transport and metabolism. Immediately after T.sub.4 is released into circulation, it is nearly completely bound to the three thyroxine binding proteins: albumin, prealbumin, and principally, TBG. The unbound fraction of T.sub.4 (free T.sub.4) provides an accurate assessment of the thyrometabolic status of a patient. The concentration of free T.sub.4 is a function of both total serum T.sub.4 and T.sub.4 binding protein concentrations. Of the three binding proteins, variation in the TBG concentration will affect the total T.sub.4 concentration the most.
Unsaturated TBG levels and a free T.sub.4 index (FTI) are often used as indicators of thyroid status. FTI is conventionally obtained by calculating the product of total T.sub.4 and a T.sub.3 uptake test. Total T.sub.4 can be determined by competitive protein binding, displacement analysis, or radioimmunoassay. The method of choice for estimating unsaturated TBG levels has been the T.sub.3 uptake test.
In the T.sub.3 uptake test, an equilibrium is developed between the patient's serum, added labeled T.sub.3, and an inert exogeneous binder (separating agent) of the labeled T.sub.3. One must add a sufficient amount of labeled T.sub.3 to saturate the binding sites on the TBG after which the labeled T.sub.3 that is unbound is adsorbed by the separating agent and counted. Therefore, when the endogeneous thyroxine level is increased, as in hyperthyroidism, serum TBG is relatively saturated and the T.sub.3 uptake will be high. Conversely, in the hypothyroid state, where thyroxine output is low, the labeled exogeneous T.sub.3 will bind to the relatively unsaturated TBG yielding a low T.sub.3 uptake.
The major variations in T.sub.3 uptake methodology have been centered around the chemical nature of the separating agent. Ion exchange resins, hemoglobin saturated charcoal, Sephadex G-25, nylon, inorganic crystalline materials, T.sub.3 antibody immobilized on the walls of polymer-coated test tubes or particles and T.sub.3 antibody immobilized on certain resins, for example, all have been used as inert binders of labeled T.sub.3. See, generally, U.S. Pat. No. 4,225,412.
U.S. Pat. No. 4,032,626 describes a process for determining the concentration of TBG in a fluid sample using radiolabeled T.sub.4 and anti-TBG antibodies immobilized on insoluble support materials. The sample is incubated with the labeled T.sub.4 and the immobilized anti-TBG antibodies. The concentration of TBG can be determined by separating the labeled composite from the unbound labeled T.sub.4 and counting the bound labeled T.sub.4.
It is the purpose of this invention to provide a novel method for determining thyroxine uptake of a sample serum for use in determining the level of unsaturated TBG and free thyroxine index of the sample serum.